Transient voltages or spikes can be very detrimental to electronic equipment commonly found in households, businesses, and communications networks. Even though each of such spikes may be of a very short duration, a cumulative effect of many such spikes may harm components of the electronic equipment resulting in premature failure of the equipment.
One method of suppressing voltage surges is to use a surge arrestor, or suppression, circuit incorporating solid state surge suppression elements such as metal oxide varistors, commonly referred to by the acronym MOV. The energy of the surge is then dissipated by the MOV instead of the attached appliances or other circuitry. Such an arrangement works well until the MOV reaches the end of its life. The MOV averts failure primarily by entering into a low resistance mode. However, in such a mode, the MOV generates a large quantity of heat, and may ignite a fire or, alternatively, may short line voltage to ground. Such a shorting may cause line protection elements, such as a fuse or a circuit breaker, to blow or trip, respectively, thereby disabling any downstream circuits or appliances powered by the line voltage. Any disabling of downstream equipment, especially in commercial applications, can be disastrous for the user. For example, the disabling of telephone circuitry from operation can result in the loss of millions of dollars of revenue, and the disabling of life support equipment in a hospital may result in a loss of life. It can be appreciated, therefore, that if an MOV enters into a low resistance mode, it may result in catastrophic consequences. Furthermore, every time that an MOV enters into a low resistance mode, the probability of catastrophic consequences occurring increases. Therefore, it is generally desirable to replace an MOV after it has once entered into a low resistance mode, to thereby avert a possible catastrophic failure. Unfortunately, it is difficult to always know when an MOV enters into a low resistance mode so that it may be replaced. While it is difficult to know when an MOV enters into a low resistance mode, it would be desirable to know when an MOV does fail, and for the effect of such failure on a previously protected circuit to be minimized.
Surges in a commercial environment are typically suppressed on the AC voltage lines of primary and secondary AC distribution panels to protect sensitive equipment which receive power from the such distribution power panels. Such surge suppression is essentially mounted in parallel to electronic equipment and provides an easy or quick path for lightning and transient surges to divert to ground instead of damaging sensitive electronic equipment that often comprise electrically sensitive semiconductor chips. Surge suppression circuitry mounted in such a manner generally comprises our basic states of operation:
1. Normal State--There is a high impedance path between ground and AC power. Current leakage is in the range of microamps. Heat is not an issue. The voltage level applied to the suppression circuitry is below a turn-on voltage of the surge suppressor.
2. High Voltage Transient Turn On State--A very low impedance path between a voltage source and ground occurs for micro-seconds, providing a low impedance path for transient energy. Transient surge voltage must exceed the turn on level of the suppression circuitry for this state to occur. Heat is a byproduct of diverting a large amount of transient energy to ground even though the impedance level is low. When the surge is of a short duration, the heat is not normally destructive in nature. The total power consumed in generating heat is less than the amount required to trip or blow breaker/fuse protective circuits.
3. Permanent Failure--A very low impedance path between a voltage source and ground. The low impedance path of the surge suppressor allows a sufficiently high level of energy to blow or trip a fuse/breaker protective circuit. If protection against excessive current is not a part of the surge suppressor design, then fuses or circuit breakers found in the AC distribution panel will be blown off-line, hopefully preventing hazardous conditions such as an electrical fire from developing. Unfortunately this also turns off important electronic circuitry downstream.
4. Catastrophic Failure--A dangerous amount of heat is developed from the surge suppression device from a limited current passing through the surge suppressor to ground. This current may be limited because of outside voltage conditions (i.e., tree fallen on a power pole) where the excessive voltage is sufficient to turn on the surge suppressor for more than a few micro-seconds but still having only a limited amount of current well-below the fuse, or in-line breaker, ratings. This is an overheat condition which can also occur in a surge suppressor component that is near its end of life and that has failed toward a shorted condition ("low" but not "very low" impedance) but has not failed totally shorted. The "low" impedance to a normally applied voltage allows a limited current to flow through the surge suppression device and allows a huge amount of heat to develop within the surge suppressor component thereby causing dangerous conditions such as smoke and fire to exist.
The use of internal thermal fuses (also known as thermal switches) in the surge suppression circuitry eliminates the weakness in both steps 3 and 4 above. A thermal fuse not only acts like a normal fuse which will blow in a permanent failed condition with normal inrush current conditions, but will also sense the build-up of heat leading to a catastrophic failure of a surge suppressor component.
The encapsulation of surge suppression circuitry in epoxy, and the placement of surge suppression circuitry in a silica sand environment, rather than in an air environment, is now an industry standard among most companies that produce surge suppression devices. The primary reason is that the sudden superheating of air adjacent an overheated surge suppressor inside a sealed environment has resulted in explosions of the container incorporating the surge suppression circuitry and has ignited fires both inside and outside the container. The technique of fully encapsulating surge suppression circuitry in epoxy, however, doesn't allow any thermal expansion of the circuit components and, in catastrophic failure conditions, causes dangerous explosive conditions to exist. Silica sand is a poor conductor of heat (an insulator) but acts as a good smothering agent in case of fire inside of the surge suppression circuitry container.
Various attempts have been made in the prior art to use a thermal fuse to sense when an MOV is approaching the end of its life so that power may be removed from the MOV prior to any failure of the MOV. Typically, this is achieved by detecting heat radiated from the MOV, which heat is indicative of the MOV entering a low resistance mode. Such an approach has been found to be very unreliable for circuitry which may be subjected to a wide range of environmental mediums and temperatures. Further, humidity and barometric pressure, when the environment is air, can affect the temperature at which a radiated heat sensitive thermal fuse operates. Finally, the spacing between the thermal fuse and the MOV is critical. Mislocations or accidental dislocations of this spacing may cause the MOV to become so hot that it will burst into flames before the thermal fuse is activated to an open condition. As known to many in the art, the thermal fuse commonly used not only opens at a given set temperature such as 200 degrees F., it may have a current rating of some value such as 15 amperes. The current rating used in the industry is a function of time as well as current (i.e., it is really a power sensitive fuse). Thus such a device may not open even though a current surge in the range of thousands of amperes flows through the device if the time is very short such as a few microseconds.
As used in this application, the term "environment" will be used to refer to all factors that may affect the radiation or transfer of heat away from the surface of a surge suppression component. As is known to those skilled in the art, surge suppression circuitry may be open to the air or enclosed in a sealed container. Further the surge suppression circuitry components may be encased in a potting compound or otherwise surrounded by a dielectric other than air such as silica sand. Each of these conditions substantially affects the rate of radiant heat transfer from a heat source to an adjacent radiant heat sensor.
Also used in this application, the term surge suppression component ("SSC") will be used to refer to any device that performs the function of surge suppression (also known as surge protection) including metal oxide varistors (MOV's), carbon based suppressors, avalanche based diodes, Transorbs.TM., gas tubes, and the like.
One of the more difficult problems in physically locating a thermal fuse relative to heat-dissipating SSC's, such as an MOV, occurs when silica sand is used as a dielectric insulator inside the product. As mentioned above, silica has become an industry standard method used to minimize the risk of heat rising too fast within the closed confines of a sealed box wherein the pressures generated by the heat could cause an explosion. When silica sand is used as part of a safety technique, the heat transferring capability of the component to the environment around the component is completely changed from what it would be without the silica sand. Thus, prior art surge suppressors were required to use different designs and component placements in accordance with the specific environment the circuit would be subjected to in end use. Such a multiplicity of designs resulted in great inefficiencies in production thereby increasing end product costs. Furthermore, since silica sand is an insulator, the spacing is more critical than it is in air for radiant heat sensing of an overheat condition of the SSC.
Some prior art circuits have attempted to utilize the increased current-carrying capacity of two thermal fuses connected in parallel in a manner similar to that shown in FIG. 1. Such a circuit has been implemented by placing the two thermal fuses on opposite sides of the heat producing element (i.e., the MOV). However, this approach created many problems in accurately locating the thermal fuses with respect to the MOV such that they would both open at substantially the same time. The result was that an explosion often occurred before radiant heat opened the second thermal fuse. The location problem was exacerbated by the use of silica sand as an insulating dielectric between the MOV and the adjacent thermal fuses.
It has also been found that, when a single thermal fuse is used in series with an MOV, a voltage surge may destroy (i.e., open) the thermal fuse even though the MOV is still operational.
It would thus be desirable to provide a more reliable method of transferring heat from a surge suppression component (SSC) to a thermal fuse whereby the surface temperature of the SSC varies over a smaller range under environmental temperature extremes before an associated thermal fuse opens. It would be further desirable to have a circuit that operates in a consistent or predictable manner over a wide range of environments (such as both air and silica sand). It would also be desirable to provide a surge suppression circuit design that allows the failure of a thermal fuse component while still providing surge suppression for the connected line and associated downstream circuits or equipment.